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Polypyridyl ruthenium(II) complexes have been widely researched as promising functional molecules. We have found unique photoisomerization reactions of polypyridyl ruthenium(II) aquo complexes. Recently we have attempted to provide insight into the mechanism of the photoisomerization of the complexes and distinguish between the distal−/proximal-isomers in their physicochemical properties and functions. Moreover, polypyridyl ruthenium(II) aquo complexes have been intensively studied as active water oxidation catalysts (WOCs) which are indispensable for artificial photosynthesis. The catalytic aspect and mechanism of water oxidation by the distal-/proximal-isomers of polypyridyl ruthenium(II) aquo complexes have been investigated to provide the guided thought to develop more efficient molecular catalysts for water oxidation. The recent progress on the photoisomerization and water oxidation of polypyridyl ruthenium(II) aquo complexes in our group are reviewed to understand the properties and functions of ruthenium complexes.

The hexagonal-shaped β -Co(OH) 2 discs were electrophoretically deposited on an electrode to yield a stably adherent and uniform β -Co(OH) 2 layer. The electrocatalytic water oxidation at the β -Co(OH) 2 layer-deposited electrode was investigated to seek efficient water oxidation catalyst for artificial photosynthesis as promising energy-providing systems for the future. The β- Co(OH) 2 layer did not work as a water oxidation catalyst in a K 2 SO 4 solution at pH 7.0 when applying 1.70 V vs. reversible hydrogen electrode (RHE), although the layer was oxidized to the CoO(OH) layer. In a phosphate buffer solution (pH 7.0), the β- Co(OH) 2 layer was dissolved and oxidatively re-deposited as the Co-Pi catalyst ( Science, 321 (2008) 1072–1075.) when applying the same potential. While in a Na 2 B 4 O 7 solution at pH 9.4, the β- Co(OH) 2 layer was oxidized to the CoO(OH) layer with the hexagonal-disc-shape remained when applying 1.7 V vs. RHE, and the formed CoO(OH) layer works efficiently for electrocatalytic water oxidation in contrast to no catalytic activity in a K 2 SO 4 solution at pH 9.4. The catalytic performance of the formed CoO(OH) layer was evaluated; the overpotential (η) of 0.42 V for current generation of 100 μA cm −2 and the Tafel slope of 0.21 ∼ 0.29 V dec −1 were provided for water oxidation at pH 9.4, which are comparable with efficient cobalt-based catalyst films reported so far.

A series of ruthenium complexes with chloro-substituted bidentate ligands, proximal-[Ru(tpy)(Cl-pyqu)L]n+ [n = 1 for L = Cl, and n = 2 for L = OH2, tpy = 2,2′;6′,2′′-terpyridine, pyqu = 2-(2′-pyridyl)quinoline] were synthesized and their reversible photoisomerizations and thermal isomerizations were investigated experimentally. The crystal structures of the complexes indicated that introduction of a chloro substituent at the 4- or 4′-position of the pyqu ligand did not change the structure around the metal center from that of the non-substituted complex, proximal-[Ru(tpy)(pyqu)L]n+. In contrast, the 6′-substituted complexes had sterically hindered environments around the metal center. The ruthenium aqua complexes showed reversible photoisomerization between the proximal and distal isomers. The quantum yield for photoisomerization of the 6′-substituted ruthenium aqua complex was almost twice as large as those of the other derivatives. This is explained by weakening of the ligand field on the ruthenium center by introduction of a chloro substituent at the 6′-position. Thermal back isomerization from the distal isomer to the proximal one was observed for the 6′-substituted complex, but such reactions were not observed for the other derivatives. The steric hindrance in the 6′-substituted aqua complex enhanced both thermal isomerization and photoisomerization.

distal-[Ru(Cl-tpy)(pynp)Cl]+ (d-2Cl) (Cl-tpy = 4′-chloro-2,2′;6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), and distal- and proximal-[Ru(Cl-tpy)(pynp)OH2]2+ (d- and p-2H2O) complexes are newly synthesized and characterized to compare structures and physicochemical properties with a 2,2′;6′,2″-terpyridine (tpy) ligand derivatives of distal-[Ru(tpy)(pynp)Cl]+ (d-1Cl), distal- and proximal-[Ru(tpy)(pynp)OH2]2+ (d- and p-1H2O). The equilibrium turned out to be involved in the aquation reaction of d-2Cl to d-2H2O in contrast to observed irreversible aquation reaction of d-1Cl under the same conditions. The kinetic analysis showed that the aquation reaction of d-2Cl is slightly slower than that of d-1Cl. The stoichiometric photoisomerization of d-2H2O to p-2H2O occurs by visible light irradiation as it is for d-1H2O, and Φ (2.1%) at 520 nm for photoisomerization of d-2H2O was higher than that (1.5%) observed for d-1H2O. d-2H2O undergoes the two-step reaction involving the successive one-proton-coupled one-electron reactions of the RuII OH2/RuIII OH and RuIII OH/RuIV O redox couples, whereas p-2H2O undergoes the one-step reaction involving the two-proton-coupled two-electron reaction of the RuII OH2/RuIV O redox couple. These redox potentials of d- and p-2H2O are higher than those for d- and p-1H2O at pH 7.0 by 10 ∼ 50 mV due to the electron-withdrawing chloro substitution. The turnover frequency ( k O 2 = 6.3 × 10 − 3 s − 1 ) of d-2H2O for water oxidation was higher than that (3.9 × 10−4 s−1) of p-2H2O by a factor of 16. kO2 for d-2H2O was also 1.6 times higher than that (3.8 × 10−3 s−1) for d-1H2O, whereas kO2 for p-2H2O was 1.2 times lower than that (4.8 × 10−4 s−1) for p-1H2O.

Electrochemical water oxidation by [Ru(EtOtpy)(bpy)OH 2 ] 2+ ( 1 ) (EtOtpy = 4′-ethoxy-2,2′:6#6°6#′-terpyridine, bpy = 2,2′-bipydidine) was investigated in a homogeneous solution under weakly acidic conditions (pH 5.3). The cyclic voltammogram of a 1 aqueous solution showed that successive proton-coupled electron transfer reactions of Ru II OH 2 / Ru III OH and Ru III OH/Ru IV O redox pairs and high anodic current above 1.1 V vs SCE. Electrocatalytic water oxidation was corroborated by the bulk electrolysis at 1.5 V; the significant amount of O 2 was evolved compared with the blank during the electrolysis. Potential-step chronocoulospectrometry (PSCCS) from 0.0 V to 1.52 V vs SCE was conducted to observe the change of 1 in solution during the electrocatalysis. The in situ UV visible spectral change showed oxidation of 1 (Ru II OH 2 ) to Ru III OH and to further oxidation of Ru III OH to Ru IV O, and that the Ru IV O species mainly exists in a steady state after 200 s in the electrocatalysis. The in situ UV–vis spectral change in a reverse potential step from 1.52 V to 0.22 V vs SCE exhibited that Ru II OH 2 completely recovers by two electron re-reduction process from the steady state in the electrocatalysis. The observation of Ru IV O in a steady state suggests that a rate determining step in the catalytic cycle is oxidation of Ru IV O to Ru V O rather than the O O bonding formation by nucleophilic attack of water to Ru V O in the electrocatalysis in a homogeneous solution.

Heterogeneous water oxidation catalyses were studied as a synthetic model of oxygen evolving complex (OEC) in photosynthesis using mica adsorbing various manganese species. Distribution of manganese species formed in the oxidative dimerization reaction of [Mn(II)(terpy)2](2+) (terpy=2,2':6',2″-terpyridine) (1') with various oxidants in water was revealed. 1' was stoichiometrically oxidized to form di-μ-oxo dinuclear manganese complex, [(OH2)(terpy)Mn(III)(μ-O)2Mn(IV)(terpy)(OH2)](3+) (1) by KMnO4 as an oxidant. When Oxone and Ce(IV) oxidants were used, the further oxidation of 1 to [(OH2)(terpy)Mn(IV)(μ-O)2Mn(IV)(terpy)(OH2)](4+) (2) was observed after the oxidative dimerization reaction of 1'. The mica adsorbates with various composition of 1', 1 and 2 were prepared by adding mica suspension to the various oxidant-treated solutions followed by filtration. The heterogeneous water oxidation catalysis by the mica adsorbates was examined using a Ce(IV) oxidant. The observed catalytic activity of the mica adsorbates corresponded to a content of 1 (1ads) adsorbed on mica for KMnO4- and Oxone-treated systems, indicating that 1' (1'ads) and 2 (2ads) adsorbed on mica do not work for the catalysis. The kinetic analysis suggested that 1ads works for the catalysis through cooperation with adjacent 1ads or 2ads, meaning that 2ads assists the cooperative catalysis by 1ads though 2ads is not able to work for the catalysis alone. For the Ce(IV)-treated system, O2 evolution was hardly observed although the sufficient amount of 1ads was contained in the mica adsorbates. This was explained by the impeded penetration of Ce(IV) ions (as an oxidant for water oxidation) into mica by Ce(3+) cations (generated in oxidative dimerization of 1') co-adsorbed with 1ads.

The synthesis of silver nanoparticles (AgNPs) within the interlayer space of transparent layered titania nanosheet (TNS) films is investigated. A considerable number of silver ions (≈70% against the cation exchange capacity of the TNS) are intercalated in the TNS films using methyl‐viologen‐containing TNSs as a precursor. The silver ion (Ag+)‐containing TNS films are treated with aqueous sodium tetrahydroborate (NaBH4), resulting in a gradual color change to bright blue. Various structural analyses clearly show that crystalline AgNPs are generated within the interlayer space of the TNSs. The NaBH4‐treated films show intense and characteristic near‐infrared (NIR) extinction spectra up to 1800 nm. The stability of the AgNPs within the TNS against oxygen and moisture is also investigated, and 96% and 82% of the AgNPs remain after standing in air for 1 month and 1 year, respectively. The NIR extinctions of the AgNP‐containing TNS films are further extended by employing different preparation procedures, for example, using sintered TNS films as starting materials and irradiating the Ag+‐containing TNSs with ultraviolet (UV) light. The obtained AgNP‐containing TNS films exhibit photochemical activities in the production of hydrogen from ammonia borane under visible‐light irradiation and the decomposition of nitrogen monoxide under UV‐light irradiation.

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs within the interlayer space of semiconducting layered titania nanosheet (TNS) films was investigated here. Two types of intermediate films (i.e., TNS films containing methyl viologen (TNS/MV2+) and 2-ammoniumethanethiol (TNS/2-AET+)) were prepared. The two intermediate films were soaked in an aqueous tetrachloroauric(III) acid (HAuCl4) solution, whereby considerable amounts of Au(III) species were accommodated within the interlayer spaces of the TNS films. The two types of obtained films were then soaked in an aqueous sodium tetrahydroborate (NaBH4) solution, whereupon the color of the films immediately changed from colorless to purple, suggesting the formation of AuNPs within the TNS interlayer. When only TNS/MV2+ was used as the intermediate film, the color of the film gradually changed from metallic purple to dusty purple within 30 min, suggesting that aggregation of AuNPs had occurred. In contrast, this color change was suppressed by using the TNS/2-AET+ intermediate film, and the AuNPs were stabilized for over 4 months, as evidenced by the characteristic extinction (absorption and scattering) band from the AuNPs.

Artificial photosynthesis is expected as one of promising clean energy-providing systems in future. Development of an efficient catalyst for water oxidation to evolve O2 is a key task to yield a breakthrough for construction of an artificial photosynthetic device. Recently, significant progress has been reported in development of the molecular catalysts for water oxidation based on manganese, ruthenium and iridium complexes. The molecular aspects of the catalysts in chemical, electrochemical, and photo- or photoelectro-driven water oxidation were reviewed to provide hints to design an efficient catalyst. This review will mainly cover the related progress in the last 5 or 6 years.

A 17.9 wt.% Ni/Al 2 O 3 /alloy catalyst with porous anodic alumina support was subjected to steam reforming of methane after 700 °C steam purging and no reactivity was shown because of surface oxidation of sintered Ni particles. When 0.05 wt.% Ru doped on this catalyst, it exhibited self-activation and self-regenerative activity, resulted from the hydrogen spillover over Ru and/or Ru–Ni alloy. Moreover, a 97% CH 4 conversion was kept for 20 h over the catalyst under electrical heating pattern.

A series of plate-type metal-monolithic anodic alumina supported nickel catalysts were employed to investigate their reactivity in the steam reforming of methane reactions. After H2 reduction, a fresh 4.7 wt% Ni/γ-Al2O3/alloy catalyst provided only a short-term activity, and then deactivated quickly. By the temperature programmed reaction technologies, the oxidation of surface sintered metallic Ni particles in the SRM test was suggested to be the main reason for the catalyst deactivation. As a result, a 17.9 wt% Ni/NiAl2O4/γ-Al2O3/alloy catalyst with an interfacial NiAl2O4 layer was synthesized, which showed excellent SRM reactivity at 700 °C and this was believed to be resulted from the existence of interfacial NiAl2O4 layer which anchored the top metallic nickel particles and effectively suppressed catalyst sintering. Moreover, no unfavorable effect was evidenced over the SRM reactivity of anodic Ni catalysts when applying the electrically heated pattern. Nevertheless, for the industrialization of anodic nickel catalysts, some efforts should be made to alleviate the deterioration of anodic supports and the catalysts sintering under the SRM reactions.

Tetraphenylporphine zinc(II) (ZnTPP) was found to be adsorbed from its CH2Cl2 solution into a Nafion (Nf) film. The characteristics of the adsorption of ZnTPP into the Nf film were studied using a visible absorption spectroscopic technique. The initial rate (v0, mol cm(-2) s(-1)) for uptake of ZnTPP was saturated with increasing ZnTPP concentration (c0, M) in the solution. This kinetic profile was analyzed in terms of a Michaelis-Menten model considering preequilibrium of ZnTPP adsorption between the solution and the outer layer of the Nf film, followed by diffusion to an inner bulk region, giving a maximum diffusion reflux of v(max) = (2.2 +/- 0.2) x 10(-13) mol cm(-2) s(-1). This is different from the kinetics for the Nf/phthalocyanine zinc(II) (ZnPc) film, which gives a linear plot of v(0) vs c(0). This can be explained by the relatively slow diffusion of ZnTPP in the film compared to that of ZnPc because of steric factors: ZnTPP contains bulky tetraphenyl moieties attached perpendicular to a porphyrin ring, whereas ZnPc has higher planarity. The isotherm for the adsorption of ZnTPP into the Nf film was analyzed using a Langmuir isotherm equation, yielding an equilibrium constant of (3.6 +/- 1.1) x 10(6) M(-1) and a saturated amount of adsorbed ZnTPP of (1.8 +/- 0.1) x 10(-9) mol cm(-2), suggesting monolayer adsorption of ZnTPP on the hydrophobic polymer network interfacial with hydrophilic transport channels without significant intermolecular overlap. This is in contrast to the multilayer adsorption mode suggested for the ZnPc adsorption. The tetraphenyl moieties could prevent the stacking of ZnTPP for multilayer adsorption.

The activity of tris(ethylenediamine)ruthenium (III) complex, [Ru(en)3]3+, as a water oxidation catalyst was studied in a homogeneous aqueous solution and a heterogeneous Nafion (Nf) membrane. In the aqueous solution, the apparent catalytic activity (kapp (s−1)) decreased monotonously with the concentration due to a bimolecular decomposition of the complex. The bimolecular decomposition of the complex was remarkably suppressed by incorporating it into a Nf membrane. An optimum complex concentration for kapp in the Nf membrane was exhibited, which was explained both by a cooperative catalysis and a bimolecular decomposition of the complex. The kapp in the Nf membrane was analyzed in terms of an intrinsic catalytic activity (kO2 (s−1)) of the complex, a cooperative catalysis distance (rco (nm)) and a critical decomposition distance (rd (nm)) between them based on intermolecular distance distribution to obtain the kO2=8.5×10−5 s−1, rco=1.44 nm and rd=1.07 nm. The results in the [Ru(en)3]3+ system were compared with those obtained in the [Ru(NH3)6]3+ system.

Electrocatalytic water oxidation was carried out using a basal-plane pyrolytic graphite (BPG) electrode coated with electrodeposited Platinum black (Pt-black) adsorbing [Ru(NH 3 ) 5 Cl] 2+ . The amount of O 2 evolved ( V O 2 , mol h −1 ) remarkably increased by adsorbing the complex onto Pt-black. The plots of V O 2 vs. the complex amount on the Pt-black gave a sigmoidal curve, which was explained by both the cooperative catalysis by two molecules of the complex and their bimolecular decomposition. The electrocatalytic activity of the complex was analyzed in terms of its intrinsic activity, k (h −1 ), cooperative catalysis distance, r co (nm) and critical decomposition distance, r d (nm) for both the void space adsorption model (VAM) and surface adsorption model (SAM) based on intermolecular distance distribution. k =940 h −1 , r co =1.21 nm and r d =0.97 nm for VAM and 950 h −1 r co =0.82 nm and r d =0.78 nm for SAM were obtained, respectively. Remarkable high intrinsic activities are ascribed to the efficient charge transfer from the electrode to the complex attached to the Pt-black.

The activity of an electrochemical water oxidation catalyst based on pentaamminechlororuthenium(III) ([Ru(NH 3 ) 5 Cl] 2+ ) complex incorporated in an electrode-coated Nafion membrane was investigated. The complex worked as an active electrochemical catalyst, and an optimum concentration for the turnover number (TN) of the catalyst in the O 2 evolution was exhibited. The TN increased with the concentration at low concentrations, which was ascribed either to charge transfer between the catalysts or to cooperative catalysis by the complex. The TN decrease at high concentrations was ascribed to bimolecular decomposition of the catalyst. The catalyst activity (TN) was analyzed in terms of a charge transfer distance between the catalysts ( r 0 /nm), a cooperative catalysis distance by the complex ( r co /nm), and a critical decomposition distance of the catalyst ( r d /nm) based on intermolecular distance distribution. It was suggested that the activity was controlled mainly by r co and r d , and the values r co =1.26 nm and r d =0.82 nm were obtained.

The effect of carbon dioxide on corrosion rate, polarization curve and hydrogen content has been investigated for carbon steels in acetate solutions and sodium carbonate solution with a pH range of 2 to 9 at 313 K under carbon dioxide and nitrogen atmospheres. In acid solutions less than pH 6 both general corrosion and hydrogen absorption are accelerated by the existence of carbon dioxide, but not clear in the intermeadiate pHs 5 and 6. In the solutions more than pH 6 little corrosion rate is observed regardless of carbon dioxide, whereas hydrogen content is surely detected only at the existence of carbon dioxide. The anodic and cathodic polarization curves show that their reaction overpotentials are affected by carbon dioxide, whose effect changes depending upon pH. The results obtained are qualitatively explained in terms of the formation of iron carbonate and the adsorption of carbonate ions.

A first and unique preparation is reported of a WO 3 /tris(2,2'-bipyrizine)ruthenium (II) (Ru(bpy) 3 2+) composite film by electrodeposition from a colloidal solution containing peroxotungstic acid and Ru(bpy) 3 2+ that is remarkably stabilized by poly(sodium 4-styrensulfonate). The composite film clearly demonstrated multiple electrochromism. The photoexcited Ru(bpy) 3 2+ was found to be quenched completely by WO 3 in the composite film, which is responsible for an electronic interaction of Ru(bpy) 3 2+ with WO 3 . The complete quenching led to a photocharging-discharging character and a steady-state photoanodic current induced by visible light.

Adsorption of [(OH(2))(terpy)Mn(mu-O)(2)Mn(terpy)(OH(2))](3+) (terpy=2,2':6',2"-terpyridine) (1) onto montmorillonite K10 (MK10) yielded catalytic dioxygen (O(2)) evolution from water using a Ce(IV) oxidant. The Mn K-edge X-ray absorption near edge structure (XANES) of the 1/MK10 hybrid suggested that the oxidation state of the di-mu-oxo Mn(2) core could be Mn(III)-Mn(IV). However the pre-edge peak in the XANES spectrum of 1 adsorbed on MK10 is different from the neat 1 powder. The kinetic analysis of O(2) evolution showed that the catalysis requires cooperation of two equivalents of 1 adsorbed on MK10. The reaction of the [(bpy)(2)Mn(mu-O)(2)Mn(bpy)(2)](3+) (bpy=2,2'-bipyridine) (2)/MK10 hybrid with a Ce(IV) oxidant evolved O(2). However, the turnover number value was less than unity for 2/MK10, showing that 2 adsorbed on MK10 does not work as a catalyst. The terminal water ligands could be an important for the catalysis by adsorbed 1. The mechanism of O(2) production by photosynthetic oxygen evolving complex is discussed based on catalytic O(2) evolution by 1 adsorbed on MK10.